This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:                3GPP third generation partnership project        BW bandwidth        C-RNTI cell radio network temporary identifier        DL downlink (eNB towards UE)        eNB EUTRAN Node B (evolved Node B)        EPC evolved packet core        EUTRAN evolved UTRAN (LTE)        FDD frequency division duplex        HARQ hybrid automatic repeat request        LTE long term evolution        MAC medium access control        MCS modulation coding scheme        MM mobility management        MME mobility management entity        Node B base station        O&M operations and maintenance        OFDMA orthogonal frequency division multiple access        PDCCH physical downlink control channel        PDCP packet data convergence protocol        PDSCH physical downlink shared channel        PDU protocol data unit        PHY physical        PRACH physical random access channel        PRB physical resource block        RA-RNTI random access radio network temporary identifier        RB radio bearer        RLC radio link control        RRC radio resource control        RRM radio resource management        SC-FDMA single carrier, frequency division multiple access        SDU service data unit        SFm m:th (mth) subframe of a radio frame        S-GW serving gateway        TDD time division duplex        UE user equipment        UL uplink (UE towards eNB)        UTRAN universal terrestrial radio access network        
A proposed communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA) is currently under development within the 3GPP. The current working assumption is that the DL access technique will be OFDMA, and the UL access technique will be SC-FDMA.
One specification of interest to these and other issues related to the invention is 3GPP TS 36.300, V8.3.0 (2007-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 8).
FIG. 1A reproduces FIG. 4 of 3GPP TS 36.300, and shows the overall architecture of the E-UTRAN system. The E-UTRAN system includes eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME (Mobility Management Entity) by means of a S1-MME interface and to a Serving Gateway (S-GW) by means of a S1-U interface. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNBs.
The eNB hosts the following functions:                functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);        IP header compression and encryption of user data stream;        selection of a MME at UE attachment;        routing of User Plane data towards Serving Gateway;        scheduling and transmission of paging messages (originated from the MME);        scheduling and transmission of broadcast information (originated from the MME or O&M); and        measurement and measurement reporting configuration for mobility and scheduling.        
Also of interest is 3GPP TS 36.321, V8.0.0 (2007-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) Medium Access Control (MAC) protocol specification (Release 8).
Of particular interest herein is the random access procedure of the LTE (E-UTRA) system. The procedure is described in 3GPP 36.300 v.8.3.0, and its steps are shown in FIG. 1B, which reproduces Figure. 10.1.5.1-1: Connection based Random Access Procedure, of 3GPP TS 36.300. The steps shown in FIG. 1B are described in detail in subclause 10.1.5.1 of 3GPP TS 36.300.
Briefly, the UE transmits a preamble and expects a response from eNB in the form of a so-called Message 2. Message 2 is transmitted on the PDSCH and its resources are allocated on the PDCCH as for any DL message. The resource allocation of Message 2 is addressed with an identity RA-RNTI that is associated with the frequency and time resources of a PRACH, but is common for the different preamble sequences. The Message 2 contains UL allocations for the transmissions of a Message 3 in the UL (step 3 of the random access procedure).
As is stated in subclause 10.1.5.1 of 3GPP TS 36.300 with respect to the Random Access Response Message (message 2), the Random Access Response generated by the MAC on the DL-SCH is semi-synchronous (within a flexible window of which the size is one or more TTI) with Message 1. No HARQ is used, and the message is addressed to the RA-RNTI on L1/L2 control channel. The Message 2 conveys at least a RA-preamble identifier, timing alignment information, an initial UL grant and an assignment of a temporary C-RNTI (which may or may not be made permanent upon RRC Contention Resolution). The Message 2 is intended for a variable number of UEs in one DL-SCH message.
As is stated in subclause 5.1.4 of 3GPP TS 36.321 with respect to the Random Access Response reception, once the Random Access Preamble is transmitted, the UE monitors the [PDCCH] in the TTI window [RA_WINDOW_BEGIN-RA_WINDOW_END] for Random Access Response(s). The UE may stop monitoring for Random Access Response(s) after successful reception of a Random Access Response corresponding to the Random Access Preamble transmission made by the UE.
If notification of a reception of the Random Access Response is received from lower layers, the UE shall: if the Random Access Response contains a Random Access Preamble identifier corresponding to the transmitted Random Access Preamble (see subclause 5.1.3) the UE shall: consider this Random Access Response reception successful and provide an indication to the higher layers; process the received Timing Alignment value (see subclause 5.2); and if an UL grant was received, process the UL grant value. If the UE does not have a C-RNTI, the Temporary C-RNTI is set to the value received in the Random Access Response message.
If no Random Access Response is received within the TTI window [RA_WINDOW_BEGIN—RA_WINDOW_END], or if all received Random Access Responses contain Random Access Preamble identifiers that do not match the transmitted Random Access Preamble, the Random Access Response reception is considered not successful and the UE shall:                if a value of a PREAMBLE_TRANSMISSION_COUNTER is less than PREAMBLE_TRANS_MAX, increment PREAMBLE_TRANSMISSION_COUNTER by 1; [compute a backoff value indicating when a new Random Access transmission shall be attempted]; and proceed to the selection of a Random Access Resource (see subclause 5.1.2).        Else, if PREAMBLE_TRANSMISSION_COUNTER is equal to PREAMBLE_TRANS_MAX the UE indicates to the higher layer that the random access procedure failed.        
A problem that arises relates to providing a flexible allocation of the Message 3 transmission while minimizing the load on PDCCH and the delay of the preamble response. The normal UL allocation, given on the PDCCH in a subframe n, points to the UL subframe n+k, where k is a parameter specified in the standard or broadcast as a part of the system information. If this definition is applied to the UL resource allocations included in the Message 2, the corresponding Messages 3 are allocated to the same UL subframe. This procedure can be problematic in some cases, especially if the system BW is small.
3GPP TS 36.321 (v.8.0.0) permits a flexible time window for transmission of the Message 2. This provides some flexibility for the Messages 3 scheduling since the eNB can delay Message 2 transmission, or it can divide the responses corresponding to the same RA-RNTI, into two or more instances of the Message 2.
However, this approach introduces at least two problems. First, a preamble retransmission by the UE is delayed because the UE needs to search for the preamble response until the end of the response window before it can conclude that its preamble was not detected by the eNB, and that the preamble retransmission is needed. Second, if the preamble responses are sent in two or more instances of the Message 2 then PDCCH resources are wasted, since each instance of the Message 2 requires its own resource allocation on the PDCCH.